This invention relates generally to pneumatic transport and distribution systems that supply multiple streams of air-entrained particles and, more particularly, to flow splitters for splitting a stream of air-entrained pulverized coal or other pulverulent material.
Pneumatic transport and distribution systems are widely used for supplying streams of air-entrained pulverized coal to a steam generator or blast furnace. In a direct-fired, coal-powered steam generator, a mill continuously grinds coal, while heated sweep air is used to dry the coal and transport the pulverized coal to a coal feed system. The mill sweep air supplies the air flow for transporting the pulverized coal to the coal feed system and also for injecting and mixing the coal with combustion air in the generator's combustors. The coal feed system divides the coal and transport air into multiple streams of air-entrained coal particles for the combustors. These streams of air and coal are generally uniform and nonvarying during combustor operation.
However, for coal to continue to be used as a source of energy, further reductions in the emissions of nitrogen oxides (NO.sub.x), sulfur dioxide (SO.sub.2), and ash particulates will be required in order to satisfy environmental concerns. The ability to burn a wider range of coals, such as high-ash, low-heat-value types of coal, will also be necessary. U.S. Pat. Nos. 4,217,132 and 4,586,443 disclose a two-stage pulverized-coal combustor that is designed specifically to provide performance improvements in these areas. The two-stage combustor includes a precombustor and a slagging combustor. The precombustor utilizes between 20% to 45% of the coal flow to provide a hot flow of combustion products and air to the slagging combustor, where they are combined with the remainder of the coal flow. In the slagging combustor, all of the coal is gasified under fuel-rich conditions, with a high percentage of the coal ash being removed as liquid slag. These fuel-rich conditions help reduce nitrogen oxide emissions, while high combustion intensities and mixing control help reduce carbon monoxide emissions. Limestone is introduced at the combustor exit to generate a highly-reactive calcined lime which is captured and used in a back-end spray-dryer absorber system for removal of sulfur dioxide emissions.
This highly-efficient two-stage combustor requires a coal feed system that can provide good coal flow stability, since combustion intensities are in excess of four times the level of conventional combustors. The coal feed system must exhaust some of the transport air prior to burning, since injecting all of the sweep air into the combustor degrades its slagging capability and emissions control. The coal feed system must also provide a variable coal split between the precombustor and slagging combustor to accommodate any changes in combustion air temperature. Accordingly, there has been a need for an improved coal feed system for use with this highly-efficient two-stage combustor. The present invention clearly fulfills this need.